This invention relates to vacuum circuit interrupters and particularly to such devices having vapor shields.
Shields are commonly used in vacuum interrupters to condense vapors produced by arcing between the contacts. Examples may be found in Cherry U.S. Pat. No. 4,020,304, Apr. 26, 1977; Wayland U.S. Pat. No. 4,553,007, Nov. 12, 1985; and Santilli U.S. Pat. No. 4,574,169, Mar. 4, 1986, which disclose various structures and compositions for such shields.
Canadian Pat. No. 693,827, Sept. 8, 1964, is directed to a vacuum interrupter wherein at least the arc supporting region of at least one of the contacts of the device has distributed throughout its substance a metal chosen from the group consisting of titanium, thorium and zirconium; such metals being referred to as high affinity metals in providing gettering of gases generated by the operation of the device. The Canadian patent also mentions that in addition to the contacts, any surfaces in the switch which are likely to have arcs formed to them, such as a shield, can also incorporate a reactive metal to provide gas fixation.
Lempert et al. U.S. Pat. No. 3,592,987, July 13, 1971, relates to gettering action in vacuum circuit interrupters by utilizing an active gettering material, such as titanium, tantalum, colombian, zirconium, tungsten or molybdenum incorporated in the electrode structures or other interior elements of the device where such materials are incorporated as filaments, or rods, either disposed randomly or in parallel alignment in a matrix of good conducting material.
Shields in present vacuum interrupters are, at least in the part vapor impinges upon, typically of copper or copper-chromium alloys formed from particles of electrolytically produced metal that are subjected to powder metallurgy techniques. The particle size and processing achieve shields with high density, at least 90% or more, and low porosity, as has been generally preferred for good conductivity and long life.
The present invention is directed to vacuum interrupters having improved vapor shields so as to have enhanced gas adsorption capability. While gettering materials may be and preferably are used in the composition of the shield, a further improvement as compared to known shields is to construct the shield with a high porosity with intercommunicating voids to the surface. This gives the shield a tremendous absorption capability for any impinging hot vapors.
In particular, copper may be the principal constituent of the shield and may be provided with high porosity by powdered metallurgical techniques to a porosity of at least about 10%. The starting material may be electrolytic copper in particles of relatively small size, e.g., less than 10 microns or about 5 to about 10 microns. Furthermore, an additive of boron can be provided to limit the growth of the copper grains during sintering. The amount of the boron may be, for example in the range of from about 3% to about 5% by weight of the finished composition.
It is beneficial to have as an additive to the composition a quantity of titanium to getter hydrogen. Other metals such as zirconium may be used in addition or alternatively for this purpose. An additional gettering metal or metals may be, for example, in the range of from about 3% to about 5% by weight of the finished composition.